I've been running several test with search/lookupfields vs. suiteql for some record types recently. I like SuiteQL as it is easier to work with and the code ends up cleaner, but the performance difference in some cases can't be ignored. Here's the result of a simple function to get some fields for a single employee is many times slower with suiteql: (seven columns, two use BUILTIN.DF)

Execution Time Summary (SuiteQL): {"Total Execution Time":"718 ms","Average Execution Time":"143.2 ms","Max Execution Time":"298 ms","Min Execution Time":"90 ms"}
Execution Time Summary (search.lookUpFields): {"Total Execution Time":"29 ms","Average Execution Time":"5.6 ms","Max Execution Time":"13 ms","Min Execution Time":"3 ms"}

Worth to consider when building wrappers for SuiteQL instead of using lookUpFields.

How's the performance like with a LEFT JOIN instead of BUILTIN.DF?

@Shai Coleman I removed those columns completely and it did improve the execution time a little bit, bit it was still around 10x slower.